It took me hours trying to decipher this three words... with these three words decoded by me initially as "塩米粉" (Salty rice noodles) for hours... even after I read from official sources what these words are in Chinese... 8-O lol... (Though no disrespect intended to those who sacrificed themselves to complete the construction of this tunnel.)

Apparently, these three characters engraved in cursive script reads "制天險" (Overcoming nature's obstacles) from right to left. If you read it the conventional way (the convention today) of from left to right, you would get "險天制."

Yet, if "險天制" still looks more like "塩米粉," it's because the characters are in cursive script rather than in standard script. The knowledge of them characters are in cursive script didn't help me a bit since, though the cursive script is also Chinese writing, it's as Greek as Greek to me.

So... let's break it down step by step like what I did...

The cursive script of 險 (the obstacles part or risk) is as the following... which now looks more similar to the characters on the left side.

The cursive script of 制 (the overcoming part) is as the following... which now looks more similar to the characters on the right.

As for 天 (the nature part), the character in the middle, which is difficult to see because of the dangling plants from above, it looks like the following in cursive script.

So, let me put these three characters in the right order for you horizontally...

Though still looking mighty like 塩米粉, it does now look more like the following...

And... don't forget to read the characters from right to left... then you get... 制天險

So it is... how I overcame my "salty rice noodles" to reach "overcoming nature's obstacles."